One thing to bear in mind is that the ambassador to France does
not need to be a particularly adept foreign policy professional.
It's not hard to maintain good relations with France. That
being said, as the chief of a major publication, there's no
reason Wintour wouldn't be as qualified as anyone else.

This isn't the ambassadorship to China, Iraq, Libya or any sort
of country where diplomatic relations requires training.

Rewarding high profile donors with these positions is a classic
way to reward well connected supporters. Indeed, a
lot of the skills needed to raise that kind of scratch —
graciousness, charisma, personality — are the exact kind of
skills needed to serve as Ambassador to a friendly nation.